The invention relates to a process for the production of Pb.sub.x Mo.sub.y S.sub.z Chevrel-phase compounds, wherein x=0.9 to 1.2, y=6.0 to 6.4, and z=7 to 8, and more particularly to a process for the production of large blocks of Pb.sub.x Mo.sub.y S.sub.z Chevrel-phase compounds including the steps of subjecting blended powders of these elements and/or of the corresponding metal sulfides to hot isostatic pressing at a temperature ranging from 800.degree. C to 1200.degree. C and a pressure in excess of 10 MPa (&gt;100 bar). 2. Description of the Related Art
Production of superconducting wires from ternary superconducting phases, for example, in the form of metal-Mo-chalcogenides, i.e., so-called Chevrel-phase compounds, has been proposed from samples of Pb.sub.x Mo.sub.y S.sub.z (hereinafter referred to as PMS) prepared either from pre-reacted powders or from a mixture of powders of Mo, MoS.sub.2, and PbS by a hot pressing technique (D. Cattani et al., 9th International Conference on Magnet Technology, Zuerich, Switzerland, Sept. 9-13th, 1985, issue MT-9 1985, pages 560-563). Optimal conditions for hot pressing were reported to be a temperature of 1100.degree. C, a duration of one hour, and a pressure of 240 MPa. These conditions were expected to produce homogeneous and dense PMS samples with a grain size of about 1 micrometer and a density of greater than 96 % of the theoretical density. Small amounts of gallium and gallium arsenide, respectively, were added to the ternary phase in order to improve the superconducting parameters of the PMS. PMS wires of several hundred meters in length were produced using, as a matrix for the PMS wires, a combination of a steel jacket and a molybdenum jacket for which the thermal expansion of the matrix matches that of the PMS and which reduces the typically occurring decline of critical current density measured for PMS samples
The Cattani et al. publication mentions, however, that use of a mixture of powders of PbS, MoS.sub.2, and Mo under the previously mentioned optimal conditions or even at a hot pressing temperature of 1500.degree. C, did not achieve a complete reaction of the materials to PMS. Cattani et al. used a modified, hydraulic, uniaxial press in which they heated the pre-reacted powders and the powder mixture, respectively, up to the desired temperature. A further disadvantage of the reference's method lies in the fact that uniform density can not be achieved with quantities of powder larger than that necessary for the production of small individual pellets. Uniform density of substantially larger blocks of PMS, however, is an absolute requirement for the production of a superconducting wire that has good superconducting properties.
Seeber et al., IEEE Trans. Mag., MAG-19, page 402 (1983) describes a process which includes a subsequent annealing at temperatures around 1000.degree. C to recover the original superconducting properties of the PMS phase.
In another process (Y. Kubo et al., International Cryogenic Materials Conference, August, 1985, Boston, U.S.A.; to be published in Adv. Cryo. Eng., Vol 32 (1986), a mixture of powders of Mo, Pb, and MoS.sub.2 was hydrostatically pressed into pellets (cold pressing), after which the pellets were introduced into a tantalum pipe. The tantalum pipe was then introduced into a stabilizing copper pipe and the whole arrangement was drawn into a wire with a diameter of 1.05 mm at room temperature. As a barrier against undesirable behavior by sulfur (sulfur sealing), molybdenum, niobium, and silver were proposed in addition to tantalum, however, tantalum was selected because it has excellent cold working properties. The wire in its final dimensions was then heat-treated for two hours at 1000.degree. C to obtain a PMS Chevrel-phase. For such a concluding heat-treatment, hot isostatic pressing (HIP) at a pressure ranging from 100 to 200 MPa and a temperature ranging from 850 to 1050.degree. C were proposed for a duration of from one to four hours.
In these latters two processes, grain growth occurs, but the disadvantages of grain growth can be limited by keeping the final dimensions of the PMS filaments 1 micrometer or less. Further, molybdenum has poor deformation properties, such that homogeneous distribution of this element was not achieved in the filaments. This has deleterious effects on the critical current density of a long wire.
The most import representative of Chevrel-phase compounds is PbMo.sub.6 S.sub.8, which is of the utmost technical interest as having the highest known critical magnetic field of ca. 60 Tesla and which is anticipated to be of paramount importance as a future superconductor for the production of the highest fields. All previously known processes for the production of Chevrel-phase compounds, however, have been only feasible on a laboratory scale, i.e., for starting material quantities of around 100 g, and have had only moderate success. Economical translation of these laboratory processes into production scale processes face several obstacles, including production problems during the deformation of a cylindrical block or pipe filled with Chevrel phase Pb.sub.x Mo.sub.y S.sub.z into a multifilament wire, degradation of the superconducting properties as a consequence of the mechanical deformation, and increasing energy requirements for scaling up to increased quantities of starting materials. Improvement of a multifilament superconductor includes increasing the number of filaments, reducing the final diameter of the filaments, and developing a matrix which stabilizes the critical current density. In addition, grain size in the filaments has to be reduced to increase the current-carrying capacity of the filaments.
Further, during the treatment of increasing quantities of starting materials, a high density of the Chevrel-phase blocks, an even, predominantly uniphasic composition, and a uniform density throughout the blocks are no longer guaranteed.